This invention relates to apparatus for feeding alternate polarity pulses to a load. The need for such apparatus arises in the construction of electromagnets in particle accelerators and FIG. 1 of the accompanying drawings shows a pulse train required for a particular accelerator.
Referring to FIG. 1 it will be seen that each pulse has a leading edge where the current rises from zero to 2500 amps within a period of 2 microseconds and then falls back to zero at a substantially constant rate during the next 3 milliseconds. Alternate pulses, of which only two are shown in FIG. 1, require the current to flow in opposite directions. Because of the high switching speeds and the high current values involved one is obliged, for applications of this type, to use switches which happen to need electrical signals to operate them. Thyratrons, thyristors and ignitrons are examples of such switches. They are usually, though not necessarily, unidirectional.
Initial consideration, by the inventor, of a method for producing pulses like those of FIG. 1, was based on a circuit as shown in FIG. 2 where the load, i.e. the electro-magnet, is schematically shown by the inductor L1.
Referring to FIG. 2, two 20 kV power supplies P1 and P2 of opposite polarity maintain opposite charges on respective capacitors C2 and C3. Two thyratrons V2 and V3 are closed at times shown on FIG. 1 by timing circuitry (not shown). These thyratrons are only maintained closed by the current that flows through them and so they open spontaneously after the end of each pulse at the times also shown on FIG. 1.
The timing circuitry also closes switches V1 and V4 at times, also shown on FIG. 1, when the current in load L1 has reached or just passed a maximum. When the switch V1 or V4 closes, stored current in an associated inductor L2 or L3 flows in the direction shown by the arrows through L1 to form the ramp-shaped trailing edge of approximately constant slope as shown on FIG. 1; this being required by the accelerator for reasons which will not be described since they are not of relevance to this invention.
Circuitry for storing current in the inductors L2 and L3 is not shown but could for example comprise a capacitor connected across a power supply and a switch for discharging the capacitor through the inductor L2 or L3 at appropriate times before each pulse.
Diodes D1 and D4 are included to prevent current flowing from L2 and L3 whilst the latter inductors are being energised. Capacitors C1 and C4 are included to reduce any discontinuity in the current through inductor L1 when the switches V1 and V4 are closed.
A problem of the circuit shown in FIG. 2 is that, whenever switch V2 or V3 is closed, the other switch V3 or V2 experiences a voltage transient (i.e. a fast voltage step) of 20 kV which is liable to cause that switch to close. When this happens a very large current flows from C3 to C2 or vice versa and no current flows through L1. This very large current can damage the switches These problems are so serious that the circuit shown in FIG. 2 is impracticable for certain applications.